ZETA-POTENTIAL OF SILVER ABSORBING THIOBACILLUS-FERROOXIDANS

Thiobacillus ferrooxidans was grown in shake flask cultures, using pow dered sulphur as the energy source. Growth of cultures was followed by monitoring the pH of the medium. Low levels (10ppm) of soluble silver nitrate (AgNO3) were introduced to some cultures, where it was absorb ed by the bacteria. Silver absorption resulted in a lag time in the gr owth phase, but not a decrease in the subsequent growth rate, and a ch ange in the colour of the culture from bright sulphur yellow to dark g rey. Zeta potentials for both these silver-loaded bacteria and a contr ol culture were measured using a particle microelectrophoretic techniq ue. The zeta potentials of the control bacteria were found to be consi stent with those recorded by other workers, with an IEP at pH 2.4. Zet a potentials of silver loaded bacteria were found to be very different exhibiting an IEP below pH 2.0, a minimum zeta potential of -32mV at pH 7.0, and an increase in zeta potential at higher pH conditions. The zeta potential of pure silver sulphide was also measured, and found t o have an IEP similar to that of the control culture. Between the IEP and pH 5, the zeta potential of the silver loaded bacteria is close to the sum of the potentials of the control bacteri and the pure mineral . It is postulated that the enhanced zeta potential is due to the addi tional surface potential of the silver sulphide precipitates. The resu lts raise the possibility of selective recovery of silver-absorbing ba cteria, and a novel silver recovery system. (C) 1998 Elsevier Science Ltd. All rights reserved.